For a wide variety of areas of application, pulverulent substances are required which have extremely low particle sizes of less than about 20 microns (.mu.m). It is frequently problematic in the case of fine-particled powders of this type that these materials are difficult to handle and, in particular, problems occur with respect to their flowability. Another particular problem is the production of coatings using these fine-particled powders. One example of such an area of application is the sector of catalyst technology, in which the active substances, in addition, have to be introduced into fine pores in the support materials.
A further important area of application for fine-particled pulverulent substances is that of powder coatings, which are gaining increasing importance.
Conventional coating procedures, in which colored pigment particles dispersed in a conventional liquid solvent are sprayed in the form of paint droplets onto the components to be coated, represent considerable environmental pollution because of the contamination of the exhaust air, which is concentrated during, for example, the line production of passenger cars.
In the interests of improved environmental protection, three kinds of coating have been developed for more environmentally friendly spray coating (R. Laible, Umweltfreundliche Lackiersysteme fur die industrielle Lackierung Environmentally Friendly Coating Systems for Industrial Coating!, Expert Verlag, Esslingen, 1988), namely high-solids coatings, water-borne coatings and powder coatings.
High-solids coatings are essentially none other than normal coatings which have been thickened; in other words, the emissions into the surrounding air of solvents of which some are harmful are reduced, but not eliminated. Waterborne coatings have the disadvantage that the overspray in the water has to be recovered. Powder coatings lead neither to significant contamination of the air nor to polluting contamination of water. In its current state of development, however, this process has a number of disadvantages which stand in the way of its broad application, especially in the sector of the thin coating of thin metal components of large surface area, for example vehicle bodies.
The principle of electrostatic powder coating, which has been introduced since 1965, is very simple. A powder coating comprises particles of colored powder dispersed in air which have a particle-size range of from 20-60 .mu.m with average particle sizes of from 40-50 .mu.m. When dispersed in air (fluidized), such powder coating particles have the flow characteristics of a fluid. This property is the reason why they can be used in spray painting. They are charged in special electrostatic spray guns and sprayed onto an earthed workpiece to which they adhere electrostatically. Particles of powder coating which have been sprayed past the workpiece and have not been deposited are suctioned off, separated from the air, screened and re-used.
The advantages of powder coating are consequently that virtually no environmental pollution occurs and that the utilization of material which can be achieved, by dry suction and separation, is very good, since overspray is recovered without troublesome residues.
The ability to utilize this powder coating technology is, however, restricted to the application of relatively thick coats of at least 70 .mu.m since, in order to have a smooth surface, the coat must be at least about 70 .mu.m thick, and it is not possible to process relatively fine particles using this process without encountering problems.
Known powder coatings belong to a class of bulk material which is capable of holding air. Its characteristics are as follows: a bed of particles of material--cracking catalysts being a typical example--expands on fluidization (fluidized bed) distinctly above the minimum fluidization, before air bubbles start to form in the fluidized bed. If the supply of air is cut off suddenly, then the bed collapses slowly at a rate of from 0.3-0.6 cm/s, corresponding to the empty-tube velocity in the suspension phase.
A type of bulk material comprising particles below about 20 .mu.m, which includes materials which are noticeably cohesive, i.e. in which the adhesive forces between the particles significantly exceed the other possible forces in the fluidized bed, namely weight and flow resistance, permits more or less poor fluidization only with the additional use of, for example, mechanical stirrers, although hardly achieving true dispersion of the particles.
Powder coatings have a solids density of about 1500 kg/m.sup.3. At atmospheric pressure, the density of air is negligible. Only a particle-size distribution of the powder coatings within the range between 20 .mu.m and 60 .mu.m guarantees that powder coatings will be in the region of the excellent fluidization properties required. The consequence of this lower particle-size limit is, therefore, that it is not possible to go below certain minimum layer thicknesses, and that high surface quality, leveling and gloss are only achieved at layer thicknesses &gt;70 .mu.m, i.e. with high paint consumption and thick coats of paint.
A significantly broadened range of applications for the known powder coating technology in order to give thinner coatings of high quality would necessitate powder coating particles of an unsuitable bulk material type containing particles below about 20 .mu.m. Prior-art powder coating technology is therefore denied access on physical grounds to desirable low particle size ranges. Important areas of application--for example catalyst production, in which the active substance is introduced into the pores of a support substance--are also closed to this technology.
Moreover, present-day powder coating technology is restricted to a number of selected binder/curing agent systems because, for example, reactive systems are not accessible to powder coating technology. For instance, it is not possible to formulate powder coatings as two-component systems in which the curing agent and the binder are stored separately and are mixed only shortly before application. In fact, to obtain homogeneous powder coatings, specific process steps are necessary in the production of the powder coatings. Conventionally the powder coating is first extruded and is only then ground, so as to guarantee a homogeneous distribution of the individual components of the powder coating. In contrast, it is not possible to produce a powder coating having homogeneous distribution of the individual components simply by dispersing them. Consequently, it is not possible to produce powder coatings at the actual premises of the user from a binder component and a curing-agent component. It is for this reason that it is not possible to formulate powder coatings which contain components which are reactive at ambient temperature, for example OH group-containing binders and curing agents which are reactive at ambient temperature and are based on free isocyanate groups.
In addition, the extrusion step which is necessary in the conventional processes for the production of powder coatings is time-consuming and cost-intensive. Furthermore, in the case of reactive systems, it is quite possible for problems to occur during extrusion (premature reaction), once again necessitating appropriate and laborious measures.
A further reason why powder coating technology is restricted to a number of selected binder/curing agent systems is that the present-day powder coatings must have glass transition temperatures Tg of &gt;40.degree. C., so as to ensure the resistance to agglomeration of the powder coatings. Indeed, the powder coatings are in general capable of being stored at ambient temperature (generally 25.degree. C.) and of remaining free-flowing if the glass transition temperature of the powder coatings is above the ambient temperature.
The technical problem on which the invention is based, starting from the fact that powder coating represents the only truly environmentally friendly coating process, and in view of the above-described limitation of present-day powder coating technology, is to broaden the scope of application of powder coating technology. Hence it should also be possible to produce relatively thin coatings with a very high surface quality and hiding power in combination with perfect leveling and gloss, without causing any new kind of environmental pollution. In addition it is intended to provide powder coatings based on an expanded range of binder/curing agent. A particular intention is to enable the formulation of powder coatings on the basis of reactive systems, i.e. binder/curing Agent systems which are reactive even at ambient temperature. Finally, it is intended to open up completely new areas of application to powder coating technology, for example the sector of catalyst production.
A process solving this problem is characterized along with developments thereof in the patent claims. Also indicated in the patent claims is a grinding process for the production of the powder to be employed in accordance with the invention, and a powder suspension which is suitable for the powder coating process.
Finally, the invention relates to the application of this process to the production of catalysts.
It is surprising and was unforeseeable that the suspension of various finely divided materials in gas liquefied under a pressure of not more than 20 bar would give suspensions which permit problem-free management and can be employed in a very wide range of sectors. For instance, the suspension according to the invention in the gas liquefied at a pressure of not more than 20 bar can be employed advantageously for all materials which can readily be suspended in the liquefied gas.